1. Field of the Invention
The present invention relates to a method and related communication device in a wireless communication system for improving uplink transmission, and more particularly, to a method and related communication device for improving uplink transmission of transmission time interval bundling.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs). The radio protocol stacks of the E-UTRAN is given including a radio resource control layer (RRC), a packet data convergence protocol layer (PDCP), a radio link control layer (RLC), a media access control layer (MAC), and a physical layer (PHY).
In order to improve uplink (UL) coverage of LTE at cell edge, transmission time interval (TTI) bundling is introduced. Activation or deactivation of TTI bundling is controlled by an RRC command and a parameter TTI_BUNDLE_SIZE, which provides the number of TTIs of a TTI bundle, is 4. Within a TTI bundle, hybrid automatic repeat request (HARQ) retransmissions are non-adaptive and are performed without waiting for feedback of previous transmissions. A HARQ feedback, e.g. positive-acknowledgement (ACK)/negative-acknowledgement (NACK) information, for a TTI bundle is only received for the TTI corresponding to the TTI_BUNDLE_SIZE, which is the last TTI of the bundle. A retransmission of a TTI bundle is also a TTI bundle. Note that, for transmission of an uplink message containing the a cell radio network temporary identifier (C-RNTI) MAC control element or an uplink message including a common control channel (CCCH) service data unit (SDU) during random access, TTI bundling does not apply.
A HARQ entity at a UE maintains a number of parallel HARQ processes allowing transmissions to take place continuously while waiting for the HARQ feedback on the successful or unsuccessful receptions of previous transmissions, and also carries new transmission parameters, e.g. a new data indicator (NDI) and spectral resources including a physical resource block (PRB) and a modulation and coding scheme (MCS), etc., for each HARQ process. At a given TTI, if an UL grant is indicated for the TTI, the HARQ entity identifies a HARQ process where a transmission in the TTI should take place. Based on a physical downlink control channel (PDCCH), the HARQ entity also determines whether a retransmission is adaptive or non-adaptive and provides transmission parameters for adaptive retransmission. When an ACK for a HARQ process is received, the HARQ entity considers the HARQ process suspended and stops generating non-adaptive retransmission for the HARQ process.
For a HARQ process, an adaptive transmission, which is a new transmission or an adaptive retransmission, is performed on the resource including PRB and MCS indicated on the PDCCH, and a non-adaptive retransmission is performed on the same resource with the same MCS which is used for the last transmission attempt. Whether an adaptive transmission is a new transmission or an adaptive retransmission is recognized by the NDI provided in the HARQ entity. A transmission in which the NDI is toggled compared to the value in the previous transmission of this HARQ process is a new transmission, while a transmission in which the NDI is not toggled is an adaptive retransmission.
The UE cannot act on TTI bundling activation or TTI bundling deactivation immediately when receiving an RRC command of bundling activation/deactivation because there is an indefinite processing delay between when the UE receives the RRC command and when the UE decodes the RRC command. Due to the uncertainty of the timing of TTI bundling deactivation, a non-bundled transmission (whether it is a new transmission or a non-adaptive retransmission) may collide with a bundled transmission (whether it is an adaptive transmission or a non-adaptive retransmission).
Please refer to FIG. 1 and FIG. 2, which are timing diagrams illustrating a transmission collision between a non-bundled transmission and a bundled transmission according to the prior art. As shown in FIG. 1, there are two PDCCH UL grants. The earlier PDCCH UL grant is received during the processing delay of TTI bundling deactivation, so that a transmission corresponding to the earlier PDCCH UL grant is still a bundled transmission, which takes place at 4 TTIs after the earlier PDCCH UL grant is received. The later PDCCH UL grant is received after the processing delay, and a corresponding non-bundled transmission takes place at 4 TTIs after the later PDCCH UL grant is received. As a result, the non-bundled transmission collides with the bundled transmission. In FIG. 2, similarly, a non-bundled retransmission triggered by a NACK collides with a bundled transmission requested by a PDCCH UL grant.
In addition to the processing delay, another reason for the transmission collision is the limited processing power of the eNB. When TTI bundling is activated, the eNB may assign colliding UL grants, which result in the transmission collision, carelessly, or report HARQ feedbacks successively and is not aware of the transmission collision. Besides, when the traffic load of the eNB is heavy, the eNB may have no way to avoid colliding UL grants in time even though it is aware of the collision. For this reason, a bundled transmission (whether it is an adaptive transmission or a non-adaptive retransmission) may collide with another bundled transmission (whether it is an adaptive transmission or a non-adaptive retransmission), which is illustrated in detail as follows.
Please refer to FIG. 3, which is a timing diagram illustrating transmission collisions between bundled transmissions according to the prior art. As shown in FIG. 3, a transmission before TTI bundling is activated is transmitted unsuccessfully and thus the UE receives a NACK. The bundled non-adaptive retransmission is triggered by the NACK and takes places at 8 TTIs after the previous transmission. At the same time, after TTI bundling is activated, the eNB assigns PDCCH UL grants frequently without considering the possible transmission collision. As a result, these bundled transmissions denoted as AAAA, BBBB, . . . , and FFFF in FIG. 3 collide with the non-adaptive retransmission triggered by the NACK. Moreover, these bundled transmissions requested by the PDCCH UL grants also overlap with each other.
Please refer to FIG. 4 and FIG. 5, which are timing diagrams illustrating a transmission collision between bundled transmissions according to the prior art. As shown in FIG. 4, two colliding PDCCH UL grants cause a transmission collision. A bundled adaptive transmission requested by an earlier PDCCH UL grant overlaps another bundled adaptive transmission requested by a later PDCCH UL grant at the last two TTIs. As shown in FIG. 5, a transmission collision is caused by two NACKs. A bundled non-adaptive retransmission triggered by an earlier NACK overlaps another bundled non-adaptive retransmission triggered by a later NACK at the last TTI.
In a brief, for the UE, the transmission collision on TTI bundling is resulted from the indefinite timing of TTI bundling deactivation and the uncertainty that the eNB may not deal with the timing of PDCCH UL grant assignment and HARQ feedback carefully. Note that, like the PDCCH UL grant does, the transmission requested by a persistent UL grant, e.g. a semi-persistent scheduling (SPS) resource, may also collide with another transmission.